[llvm] [AMDGPU] Make sort ordering in `AMDGPURewriteAGPRCopyMFMAImpl::eliminateSpillsOfReassignedVGPRs()` strict weak. (PR #162493)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-amdgpu

Author: weiwei chen (weiweichen)

<details>
<summary>Changes</summary>

To fix the crash mentioned in 
https://github.com/llvm/llvm-project/issues/162490

- [x] `sort` needs the comparator with strictly weak ordering, however current logic doesn't meet the [**Antisymmetry**](https://tanjim131.github.io/2020-05-22-strict-weak-ordering/#:~:text=Almost%20all%20C++%20STL%20containers,the%20person%20with%20greater%20height.) requirement with 

```
sort 0x561ecd3d3db0,0x561eaba91d10  25
  weight 0.000000e+00,0.000000e+00
  size 650370,662754
  slot 732,733
```

Make the comparator logic strict weak order.

---
Full diff: https://github.com/llvm/llvm-project/pull/162493.diff


1 Files Affected:

- (modified) llvm/lib/Target/AMDGPU/AMDGPURewriteAGPRCopyMFMA.cpp (+3-1) 


``````````diff
diff --git a/llvm/lib/Target/AMDGPU/AMDGPURewriteAGPRCopyMFMA.cpp b/llvm/lib/Target/AMDGPU/AMDGPURewriteAGPRCopyMFMA.cpp
index fedb694bfcc2a..caaa92999fd1a 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPURewriteAGPRCopyMFMA.cpp
+++ b/llvm/lib/Target/AMDGPU/AMDGPURewriteAGPRCopyMFMA.cpp
@@ -490,7 +490,9 @@ void AMDGPURewriteAGPRCopyMFMAImpl::eliminateSpillsOfReassignedVGPRs() const {
       return true;
 
     // Tie breaker by number to avoid need for stable sort
-    return A->reg().stackSlotIndex() < B->reg().stackSlotIndex();
+    // The ordering have to be strictly weak.
+    return (A->reg().stackSlotIndex() < B->reg().stackSlotIndex()) &&
+           ((A->weight() <= B->weight()) && (A->getSize() <= B->getSize()));
   });
 
   // FIXME: The APIs for dealing with the LiveInterval of a frame index are

``````````

</details>


https://github.com/llvm/llvm-project/pull/162493